Municipal waste has traditionally been disposed of in landfill sites. However, the environmental hazards of doing so are becoming a major concern and therefore an effort has been made in recent years to develop waste-treatment processes that reduce the volume of the waste material and the amount of potentially environmentally hazardous constituents in the treated material.
Processes that have been developed to treat waste include combustion systems, in which the waste is thermally processed with stoichiometric or excess amounts of oxygen. The process is normally carried out in air. Examples of combustion systems include: mass-fired combustion systems, refuse derived fuel (RDF) combustion systems, in which the RDF is typically burnt on a travelling grate stoker, and fluidised bed combustion.
Another method of processing waste involves using pyrolysis, i.e. pyrolysing the waste in a pyrolysis unit. The term pyrolysis means, in the field of waste treatment, the thermal processing of waste in the absence of oxygen. Generally pyrolysis processes are endothermic and so require the input of thermal energy for the pyrolysis to continue. This contrasts with combustion, which is an exothermic process, and as such does not require the additional input of heat once the combustion has been initiated. The pyrolysis process converts many of the organic constituents found in waste to gaseous, liquid and solid fractions using a combination of thermal cracking and condensation reactions. Pyrolysis generally results in three products: a gas stream, primarily containing hydrogen, methane, carbon monoxide, carbon dioxide and other gases; a liquid fraction containing a tar or oil containing acetic acid, acetone, methanol, and complex oxygenated hydrocarbons; a char, consisting of almost pure carbon, plus any originally inert material originally present in the solid waste. Pyrolysis is a process that is used in the industrial production of charcoal from wood, coke and coke gas from coal, and fuel gas and pitch from heavy petroleum fractions. However, its use in the processing of solid waste has not been successful, one of the reasons for which is that the system requires a consistent feedstock, which is difficult to obtain from municipal waste.
A third method for processing waste involves the gasification of the waste. Gasification is the partial combustion of a material, where the oxygen in the gasification unit is controlled such that it is present at a sub-stoichiometric amount, relative to the waste material. Gasification of waste containing carbonaceous components results in a combustible fuel gas rich in carbon monoxide, hydrogen and some saturated hydrocarbons, principally methane. There are five basic types of gasifier: vertical fixed bed gasifier, horizontal fixed bed gasifier, fluidised bed gasifier, multiple hearth gasifier and rotary kiln gasifier. The first three are in most common use.
Gasification, while being moderately successful in combusting the majority of waste, nevertheless produces a gas that contains uncombusted particulates, low volatility tarry species and airborne compounds. Additionally, although much of the waste is combusted to either a gas or airborne particles, the gasification process still often results in a ‘char’, i.e. a solid material that contains constituents that will not readily combust or vaporise under the operating conditions of the gasification. The char commonly contains hazardous heavy metal and toxic organic species, which must be disposed of carefully, adding to the cost of the overall waste treatment process. It will be appreciated that there is a desire to reduce the amount of solid waste that results from a waste-treatment process, and also reduce the amount of hazardous materials in the treated waste.
It has also been found that, if the gas that results from the gasification of waste (termed an ‘offgas’) is used in a gas engine or gas turbine, the airborne particulates and tarry hydrocarbon molecules have a tendency to clog the gas turbine or engine. The gas is therefore not considered to be sufficiently ‘clean’ and even if the offgas produced by the gasification were to be used, the turbine would require frequent cleaning and maintenance and/or the introduction of an additional costly cleaning stage to remove the tarry products.
There is therefore a desire for a process that will overcome, or at least mitigate, some or all of the problems associated with the methods of the prior art.